The trimming (adjustment) of resistors is a widely used procedure in the manufacture of microelectronics and electronic components, and in common design of user circuits, especially where precision calibration is desired. In principle, one trims the resistor until an observable local or global circuit parameter reaches a desired value. Resistor trimming is widespread in both manufacturing of a variety of components and instruments, and in the user community.
Several methods exist for trimming resistors, applicable at various levels of the manufacturer-to-user chain, including laser-trimming, electrical trimming, trimming by reconfiguration of resistor networks using fuses, and the use of trimpots (potentiometers) having variable numbers of resistive turns.
Electro-thermal trimming phenomena have been observed by several authors, for the trimming of a variety of resistor materials. For example, Kato and Ono (“Constant Voltage Trimming of Heavily-Doped Polysilicon Resistors,” Jpn. J. Appl. Phys. Vol. 34, 1995, pp.45-53), related experimental results to a theoretical model, whereby the instability of polysilicon as a function of applied voltage and current were explained by melting-segregation at grain boundaries in the polysilicon, modified by the temperature dependence of grain resistance. In their formulation, the resistance behavior is found to be highly non-linear, with little or no change in resistance for low power dissipation (below a certain threshold), and dramatically increased instability above a certain threshold. Also, research at Motorola by D. Feldbaumer, J. Babcock, (“Theory and Application of Polysilicon Resistor Trimming”, Solid-State Electronics, 1995, vol. 38, pp. 1861-1869) prove that polysilicon resistors, so-trimmed at higher temperature(s), exhibit excellent stability in absolute resistance, during operation at or near room temperature.
The thermal instability, (unstable resistance variation with temperature) of polysilicon resistors located on micro-machined platform suspended over cavities, is known (Canadian Microelectronics Corporation Report #IC95-08 September 1995; and O. Grudin, R. Marinescu, L. M. Landsberger, D. Cheeke, M. Kahrizi, “CMOS-Compatible High-Temperature Micro-Heater: Microstructure Release and Testing,” Canadian Journal of Elec. and Comp. Engineering, 2000, Vol. 25, No. 1, pp. 29-34.) It is known that, for a resistance element on a micro-platform, the resistance could be increased or decreased depending on the power applied through that resistance element. This is usually considered to be a disadvantage for the use of polysilicon for resistive elements where stability is important. This present invention concerns the use of this instability (or any similar threshold-dependent instability in resistive materials), to overcome any of a group of several obstacles present in the aggregate of the prior art. In particular, the material should be stable below a certain threshold of temperature or power dissipation, and relatively less stable above such a threshold, such that its resistance can be modified.
There exists electrical trimming of metal resistors based on inducing electro-migration in the resistive elements by pulsing high currents (U.S. Pat. Nos. 4,870,472, 4,606,781). This method relies on very high current density to cause the electro-migration.
There exists electrical trimming based on thermally-induced changes in resistivity of polysilicon resistors residing on a substrate (U.S. Pat. No. 4,210,996; D. Feldbaumer, J. Babcock, V. Mercier, C. Chun, “Pulse Current Trimming of Polysilicon Resistors”, IEEE Trans. Electron Devices, 1995, vol. 42, pp. 689-695; D. Feldbaumer, J. Babcock, “Theory and Application of Polysilicon Resistor Trimming”, Solid-State Electronics, 1995, vol. 38, pp. 1861-1869), or resistors made from other thermally-mutable types of materials). This method relies on the application of highly dissipated power (such as several watts), to sufficiently heat the resistors while they are on the substrate which effectively acts as a heat-sink. This in turn requires high voltage and brings the danger of damage from electrostatic discharge (ESD).
Arguably in large part to get around this problem, Motorola invented thermal trimming of a functional resistor by an auxiliary resistor which is electrically-isolated from the functional resistor. This allows functional trimming to set a parameter of a larger circuit (U.S. Pat. Nos. 5,679,275, 5,635,893, 5,466,484), with the trimmable resistor as a component, without repeated disconnection-reconnection of the functional resistor. This also allows the trimming of resistors having high resistance values, without extra constraints on the heater resistor.
The Motorola invention involves placing the resistors one over the other, separated by a very thin electrically-insulating film. This configuration is required principally because the two resistors are situated on the substrate, which acts as a heat sink. Thus, a substantial amount of power is required to be dissipated to attain the trimming temperature. Consequently, the one-over-the-other configuration is preferred in the prior art, in order to maximize the heat transferred from the heater-resistor to the functional-resistor. Any other configuration, such as side-by-side and made from the same deposited layer, would require much higher power-dissipation in the heater-resistor, which in turn would require a higher supply voltage and would unduly heat the substrate. It should be noted that the substantial power dissipated in the heater resistor must be conducted away through the insulating oxide, functional resistor, and other surrounding layers and devices.
The concept of a micro-platform or microstructure suspended over a cavity in a substrate (such as a cavity micro-machined in silicon), including electrically-resistive elements for heating and/or sensing, has been well-known in the literature for a decade or more (Canadian Microelectronics Corporation Report #IC95-08 September 1995; F. Volklein and H. Baltes, “A Microstructure for Measurement of Thermal Conductivity of Polysilicon Thin Films”, J. Microelectromechanical Systems, Vol. 1, No. 4, December 1992, p. 193, and references therein; Y. C. Tai and R. S. Muller, “Lightly-Doped Polysilicon Bridge as an Anemometer,” Transducers '87, Rec. of the 4th International Conference on Solid-State Sensors and Actuators 1987, pp. 360-363; N. R. Swart and A. Nathan, “Reliability Study of Polysilicon for Micro-hotplates,” Solid State Sensor and Actuator Workshop, Hilton Head, Jun. 13-16, 1994, pp. 119-122.). Micro-platforms with embedded resistive elements are commonly seen in micro-sensor, micro-actuator and micro-electromechanical systems (MEMS) literature since 1990 or earlier (e.g. I. H. Choi and K. Wise, “A Silicon-Thermopile-Based Infrared Sensing Array for Use in Automated Manufacturing,” IEEE Transactions on Electron Devices, vol. ED-33, No. 1, pp. 72-79, January 1986).
The concept of using a resistive heater to heat an entire suspended micro-platform or microstructure is also well-known in the literature for at least approximately a decade (C. H. Mastrangelo, J. H.-L. Yeh, R. S. Muller, “Electrical and Optical Characteristics of Vacuum-Sealed Polysilicon Micro-lamps”, IEEE Trans. Electron. Dev., vol.39, No. 6, June 1992, pp. 1363-1375; N. R. Swart, and A. Nathan, “Reliability of Polysilicon for Micro-plates,” Solid-State Sensor and Actuators Workshop, Hilton Head, S.C., Jun. 13-16, 1994, pp. 119-122; S. Wessel, M. Parameswaran, R. F. Frindt, and R. Morrison, “A CMOS Thermally-isolated Heater Structure as Substrate for Semiconductor Gas Sensors,” Microelectronics, Vol. 23, No. 6, September 1992, pp. 451-456; M. Parameswaran, A. M. Robinson, Lj. Ristic, K. C. Chau, and W. Allegretto, “A CMOS Thermally Isolated Gas Flow Sensor,” Sensors and Materials, 2, 1, (1990), pp. 17-26.)
University of Michigan has patented (U.S. Pat. No. 6,169,321), the thermally-induced modification of parameters such as resonance frequency and Q of micro-machined resonators and other micro-structures residing on a micro-platform, using a separate micro-heater, also on the micro-platform.
Thermal trimming of a thermo-anemometer-type of sensor (such as a thermal accelerometer) is known (U.S. Pat. No. 5,808,197), where temperature-sensitive metal resistors are heated until they oxidize, hence changing the resistance of the metal film. This procedure is not reversible, and can be used only at the manufacturing stage (not practical for user- or field-trimming).
A method of creating a precise resistance having little-to-no drift with temperature (so-called “zero-TCR”), based on combination (connection) of two resistors having positive and negative temperature coefficients of resistance (TCR's), with subsequent laser trimming, has also been invented (U.S. Pat. No. 6,097,276). The method involves calibration steps wherein the resistor is heated up to a predetermined temperature (T), then the T-induced resistance drift is measured, then the structure is laser-trimmed to minimize net TCR of the combined resistor, and then the process is repeated until the TCR is reduced to the desired level.
The inclusion of trimmable resistors in certain devices and applications has also been considered in prior inventions (U.S. Pat. Nos. 5,679,275, 5,635,893, 5,466,484). In particular, the use in op amps, in reference voltage sources, and in digital-to-analog convertors and analog-to-digital convertors (DAC/ADC's) has been outlined (U.S. Pat. Nos. 5,679,275, 5,635,893, 5,466,484.)
There is a need for highly accurate trimming that can exceed the accuracy achieved by laser trimming and does not require special equipment such as powerful lasers.